An A-plate is a wave plate commonly used as a retarder in an optical device. It is a birefringent material capable of manipulating the polarization state or phase of the light beam traveling through the medium. The A-plate optical retarder has a refractive index profile of nx>ny=nz, wherein nx and ny represent in-plane refractive indices and nz the thickness-direction refractive index. Such a wave plate exhibits a positive in-plane retardation (Re) as expressed by Re=(nx−ny)×d, wherein d is the thickness of the wave plate.
An A-plate having in-plane retardation (Re) equal to a quarter of a light wavelength (λ), Re=λ/4, is called quarter wave plate (QWP). Likewise, an A-plate having Re equal to half of the wavelength, Re=λ/2, is called half wave plate (HWP). A QWP is capable of converting an incident linearly polarized light to circularly polarized light. Thus, it is commonly used in combination with a linear polarizer to provide a circular polarizer in an optical device. An HWP is capable of rotating the plane of polarization by 2θ, where θ is the angle of the plane polarized light with respect to the slow (or fast) axis of the wave plate.
A-plates are commonly used in liquid crystal displays (LCDs) as compensation films to improve the viewing angles. They can also be used in an OLED (organic light emitting diode) display device. For example, A QWP is being used with a linear polarizer to provide a circular polarizer in an OLED device to reduce the ambient light reflected by OLED for improved viewing quality. These applications typically utilize the in-plane retardation provided by the A-plate for in-plane phase-shift compensation. For example, A-plate combining with C-plate is particularly useful in reducing light leakage of the crossed polarizers at oblique viewing angles. The A-plate, however, also exhibits a negative out-of-plane retardation Rth, which is defined as Rth=[nz−(nx+ny)/2]×d with a value of |Re/2| arising from its orientation. This characteristic can be beneficial when a negative Rth is desirable in an optical device. For example, in a vertically aligned (VA) mode LCD, the liquid crystal molecules in the LC cell are aligned in a homeotropic manner, which results in positive out-of-plane retardation. An A-plate, thus, can provide an out-of-plane compensation in addition to in-plane compensation in VA-LCD. In other devices, such as in-plane switch (IPS) mode LCD and OLED display, however, the Rth exhibited in the A-plate is not desirable since it can give rise to phase shift in off-axis light and lead to light leakage. Thus, there exists a need in the art to provide a positive in-plane retarder having reduced out-of-plane retardation for improved viewing angle and contrast ratio of the display.
U.S. Pat. No. 7,211,316 discloses an optical multilayer comprising a polymeric substrate and an amorphous polymer having Tg above 160° C. and positive birefringence so as to provide a total out-of-plane phase retardation of said multilayer of between −30 nm and 30 nm.